The present invention relates generally to a dye lasing arrangement such as a dye laser oscillator or dye amplifier in which a continuous stream of dye is caused to flow through a given zone in a cooperating dye chamber while the zone is being illuminated by light from a pumping beam. This in turn causes the dye flowing through the zone to lase and thereby initially produce a dye beam in the case of a dye laser oscillator or amplify an existing dye beam in the case of a dye amplifier. The present invention relates more particularly to an optical assembly for altering the cross-sectional configuration of the pumping beam such that the beam enters the dye chamber so as to more uniformly and more efficiently illuminate the lasing zone.
Lasing arrangements of the type cited above, specifically a dye laser amplifier and a dye laser oscillator, are disclosed in copending U.S. application Ser. No. 911,271, filed Sep. 22, 1984 and entitled HIGH FLOW VELOCITY DYE LASER AMPLIFIER and reference is made to this application which has been assigned to the assignee of the present application. In each of these arrangements, a continuous stream of dye is caused to flow through a given zone in a cooperating dye chamber. At the same time, this zone is illuminated by light from a pumping beam, specifically a copper vapor laser beam, in order to cause the dye flowing through the zone to lase and thereby initially produce a dye beam or amplify an existing one. In either case, in order to make the lasing operation an efficient one, it is important that the entire lasing zone be uniformly illuminated by the pumping beam or as uniformly illuminated as possible. In the arrangements disclosed in the patent application just recited, each pumping beam used enters the dye chamber through a cooperating window which is rectangular in configuration. At the same time, the pumping beam, a copper vapor laser beam in the case of the disclosed arrangements, is initially produced with a circular cross-sectional configuration. Therefore, in order to more efficiently illuminate the lasing zone within the dye chamber, it is at least necessary to conform the cross-sectional configuration of the pump beam to that of the entry window. However, the intensity profile of this beam, in circular cross-section, is typically not uniform as will be discussed below. Therefore merely altering its cross-section from one which is circular to one which is rectangular will not make its intensity profile any more uniform, although this is a first step in more uniformly illuminating the lasing zone.
In the case of copper vapor laser beams, if the effects known as radial dynamics resulting from the production of such a beam are not compensated for, the beam is produced with a "chevron" effect. This means that its intensity profile in cross-section varies between one extreme such that the intensity is concentrated in the center section of the beam and an opposite extreme such that its intensity is concentrated within an outer annulus. Even if the beam were produced without the effects of radial dynamics, it would nevertheless tend to have a non-uniform cross-sectional intensity profile. For example, the "perfect" laser beam has an intensity profile which is Gausian distributed in cross-section. Therefore, even if the "perfect" pumping beam is acted upon merely to convert its circular cross-section to one which corresponds to the rectangular configuration of the inlet window into the dye chamber, as discussed immediately above, while the entire lasing zone would be illuminated by light from the pumping beam, the zone would not be illuminated uniformly since the intensity profile of the rectangular beam would not be uniform in cross-section.
Still referring to the background of the present invention, the present comments pertain to a dye laser amplifier or oscillator pumped by a laser source such as a copper vapor laser with a round output beam. The lasing or amplifying volume is shown in FIG. 9. The dye flows in the y direction, the rectangular dye laser beam propagates in the z direction and the pump beams enter the rectangular volume from the plus x and minus x directions. The pump beams should illuminate as little dye outside the volume swept by the dye laser beam as possible. Aside from the obvious waste of pump power, there is a more important reason to do so. Pumped volume outside the swept region will spontaneously emit photons that will sap power out of the swept region. These photons will not have the desired wavelength and some of them will propagate forward through the system being amplified by succeeding amplifiers thus wasting more pump power. This "amplified spontaneous emission" will have poor wavefront quality so it can spill onto mirror and window mounts, potentially damaging them. Minimizing the deposition of pump energy outside the swept volume places three conditions on the pump beam. First, a hard aperture should be imaged into the dye cell to minimize diffractive blurring. Second, the pump beam should be collimated so the beam holds its shape as it propagates through the cell. Third, the alignment system must maintain pointing.
One tries to inject a dye beam intense enough so it can extract all of the energy deposited by the pump beam; that is, operating in the saturated gain regime. If that is the case, the output intensity is roughly equal to the input intensity plus the integral over the ray path of the pump intensity times the conversion efficiency. EQU Output(y)=Input(y)+[Eff.*Pump(y,z)]dz
Assuming that the input dye beam is relatively uniform, the output dye beam will only be uniform in y if the integral does not vary much in that dimension. If the dye beam has a relatively uniform intensity, it will be more efficient in laser chemical processes and will show improved diffraction characteristics, thus reducing transmission losses. "Hot spots" in the beam also tend to damage optics.
The previous art in transverse pumping of dye laser amplifiers with laser light consisted of transforming the round pump beam into an elliptical one whose "height" (y dimension in FIG. 9) was matched to the laser beam to be amplified, and whose "width" (z dimension in FIG. 9) was chosen to be long enough to keep the pump beams from burning the input windows. An elliptical beam fills 78% of a "best fit" rectangular dye amplifier cavity. Thus the pumped area swept out by a ray passing through the center of the cavity is much greater than that swept by a ray near the top or bottom of the cavity. A more uniform intensity dye laser beam would propagate with less diffraction loss and can have a higher average flux without damaging optics. The improvement in dye laser pumping described in this patent application involves "slicing" the laser beam into three slabs and superimposing them on top of one another in the dye cell. This greatly improves the uniformity of pumping and thus the amplified beam. A description of this will be presented in the next section.
The pump beams should be collimated in the dye cell so they maintain a constant cross-section through the cell. Furthermore, a hard aperture in the pump laser beam should be imaged into the cell. This minimizes diffraction spreading so there is as little pumping outside the swept region as possible.